This invention relates generally to implantable medical stimulators, and more particularly to implantable cardioverters and defibrillators.
Initially, implantable cardioverters and defibrillators were envisioned as operating with a single pair of electrodes applied on or in the heart. Examples of such systems are disclosed in U.S. Pat. No. 3,614,954 by Mirowski et al., in which pulses are delivered between an electrode placed in the right ventricle, and a second electrode placed outside the right ventricle. Studies have indicated that two electrode defibrillation systems often require undesirably high energy levels to effect defibrillation. In an effort to reduce the amount of energy required to effect defibrillation, numerous suggestions have been made with regard to multiple electrode systems. For example, sequential pulse multiple electrode systems are disclosed in U.S. Pat. No. 4,291,699, issued to Geddes et al., in U.S. Pat. No. 4,708,145, issued to Tacker et al., in U.S. Pat. No. 4,727,877 issued to Kallok and in U.S. Pat. No. 4,932,407 issued to Williams. Sequential pulse systems operate based on the assumption that sequential defibrillation pulses, delivered between differing electrode pairs have an additive effect such that the overall energy requirements to achieve defibrillation are less than the energy levels required to accomplish defibrillation using a single pair of electrodes.
An alternative approach to multiple electrode, sequential pulse defibrillation is disclosed in U.S. Pat. No. 4,641,656 issued to Smits and also in the above-cited Williams patent. This defibrillation method may conveniently be referred to as multiple electrode, simultaneous pulse defibrillation, and involves the delivery of defibrillation pulses simultaneously between two different pairs of electrodes. For example, one electrode pair may include a right ventricular electrode and a coronary sinus electrode, and the second electrode pair may include a right ventricular electrode and a subcutaneous patch electrode, with the right ventricular electrode serving as a common electrode to both electrode pairs. An alternative multiple electrode, simultaneous pulse system is disclosed in U.S. Pat. No. 4,953,551, issued to Mehra et al., employing right ventricular, superior vena cava and subcutaneous patch electrodes.
In the above-cited prior simultaneous pulse, multiple electrode systems, delivery of the simultaneous defibrillation pulses is accomplished by simply coupling two of the electrodes together. For example, in the above-cited Mehra patent, the superior vena cava and subcutaneous patch electrodes are electrically coupled, and a pulse is delivered between these two electrodes and the right ventricular electrode. Similarly, in the above-cited Williams patent, the subcutaneous patch and coronary sinus electrodes are electrically coupled, and a pulse is delivered between these two electrodes and a right ventricular electrode.